DIY craft question-tin can lantern?

by DIY wedding planner on August 7, 2012

I’m making tin can lanterns for my wedding. They are very easy-fill with water, freeze, punch holes. However, some of my cans bulge at the bottom.
Before anyone explains this I know why it happens-water expands, I get it. But what I don’t get is why some of the cans bulge on the bottom and some don’t. I’ve tried everything, different levels of water and flipping the cans halfway through, but about 75% of the cans buckle. Can’t use the bulging ones because they won’t sit flat.
Any ideas how I can stop this from happening?

I’m just making an educated guess here, but maybe the slug of ice is getting locked in the ribs of the can and thus can’t raise up as the lower part freezes and expands. You might try putting in just an inch of water to freeze then add another inch etc. or could you just whack the bulged bottoms back in with a hammer?